Method of testing heat exchangers for leaks



March 1, 1960 OBRIEN ETAL METHOD OF TESTING HEAT EXCHANGERS FOR LEAKS Filed July 29, 1957 COOLANT NH3 CONVERTER INVENTORS GEORGE O'BRIEN O. W. HILL BY A T TOPNEVS NIETHOD OF TESTING HEAT EXCHANGERS FOR LEAKS George OBrien, Bartlesville, kla., and Oliver W. Hill, Dumas, Tex., assignors to Phillips Petroleum Company, a corporation of Delaware Application July 29, 1957, Serial No. 674,734

6 Claims. (Cl. 23-230) This invention relates to a method of testing heat exchangers for leaks. In one aspect this invention relates to a method for detecting leaks in heat exchangers. In another aspect this invention relates to a method for determining the magnitude of a leak in a heat exchanger.

.It is common place to employ heat exchange between streams at different temperatures in chemical processes. Both direct heat exchange, wherein the two streams are brought into direct contact with each other, and indirect heat exchange are employed. In indirect heat exchange a stream of a first fluid is passed through a confined passage disposed within another confined passage through which a stream of a second fluid is passed. The familiar shell and tube heat exchanger is a well-known example of an indirect heat exchanger. Such a heat exchanger comprises a nest or bundle of tubes supported in tube sheets and disposed Within a shell. It frequently occurs that a leak develops in one of said tubes or tube sheets with the result that one of the fluids leaks into and contaminates the other of said fluids. Said leaks are frequently difficult to detect by chemical analysis, particularly when the fluids which are being passed through the heat exchanger are of similar composition. This is especially true when the fluids which are exchanging heat are gases. It sometimes happens in some processes that leaks can go undetected for a period of time sufiicient to result in considerable economic loss.

We have found that when a stream of a first gas mixture containing little or none of a readily condensible component, water vapor for example, is passed in indirect heat exchange with a stream of a second gas mixture which is at a higher pressure and which contains a higher concentration of said readily condensible component than said first gas mixture, a leak in the heat exchanger can be readily and conveniently detected by passing a sample stream of said first gasleaving the heat exchanger through a condenser and cooling it, at a pressure substantially the same as the pressure of said first gas, to a temperature substantially the same as, but not lower than, the dewpoint of said first stream of gas entering said heat exchanger. When the amount of readily condensible component:in said two streams of gas mixture is known, and .said streams are passed through said heat exchanger at known rates of flow, the magnitude of the leak can be calculated from the amount of condensate collected.

An object of this invention is to provide a simple and inexpensive method for detecting leaks in heat exchange equipment. Another object of this invention is to provide a simple and inexpensive method for determining the magnitude of leaks in heat exchange equipment. Other aspects, objects, and advantages of the invention .will be apparent to those skilled in the art in view of this disclosure.

United States Patent till " In the practice of the invention, in a process wherein ICC heat exchange with a stream of a cooling medium, In

said condenser, said sample stream is cooled, at a pressure substantially the same as the pressure of said first gas, to a temperature substantially the same as but not lower than the original dewpoint, with respect to said readily condensible component, of said cool first gas prior to entering said heat exchanger. Upon being cooled to substantially its original dewpoint, any of said readily condensible component from said Warm second gas which has leaked into said cool first gas through a leak in the heat exchanger will be condensed. Collection and observation of the amount of condensate will give a qualitative indication of the presence of a leak in the heat exchanger.

As used herein and in the claims, the dewpoint is defined as the temperature at which a given mixture of gas (or a mixture of gases) and a readily condensible component is saturated with said readily condensible component.

When it is desired to determine the magnitude of the leak in the heat exchanger, said sample stream is passed through said condenser at a predetermined rate of flow and cooled to a predetermined temperature at a predetermined pressure, and the amount of condensate collected in a given period of time is measured. Knowing the concentration of said readily condensible component in said first and second gas streams, and the amounts of said first and second gas streams passed through said heat exchanger, the magnitude of the leak in the heat exchanger can be calculated from the measured volume of condensate collected from the outlet of said condenser.

It is to be noted that said sample stream of gas is cooled to a temperature substantially the same as, but not lower than, the original dewpoint of said gas prior to its entry into said heat exchanger. It is essential that said gas not be cooled lower than its original dewpoint because otherwise part of the condensate may represent readily condensible component which was present in said cool first gas prior to its entry into the heat exchanger. The permissible AT, i.e., the difference between the temperature of said sample stream of gas exiting from said condenser and the original temperature of said cool first gas prior to entering said heat exchanger, will 'vary depending upon the magnitude of the leak and the AP, i.e., the difference in pressure between said two streams. For example, when AP is 0, AT can be within the range of 0 to about 10 F., preferably within the: range of 0 to about 5 F.-, greater than said original temperature. Table I given below illustrates the relationship between AT, AP, and the magnitude of the leak.

It is preferable that the pressure of the sample stream of gas exiting from said condenser remain substantially the same as the original pressure of said cool first gas prior to its entry into the heat exchanger. However the effect of pressure is less than the etiect of temperature. The permissible AP, i.e., the difference between the pressure of said sample stream of gas when it exits from said condenser and the original pressure of said cool first gas prior to entering said heat exchanger, will vary depending ppon the size of the leak and the AT. See Table I given below,

TABLE I Relationship between AT, AP and magnitude of leak Magnitude of Leak, Vol. Percent AT, F. AP, p.s.i. 1% 2% 3% ml. condensate/1,000 s.e.t. gas tested Any suitable cooling medium can be employed for cooling said sample stream of gas. An especially convenient cooling medium is a stream of said cool first gas which is being passed to the heat exchanger. When thus using a stream of the cool first gas at its original temperature and pressure as the cooling medium to cool said sample stream, it is impossible to cool said sample stream below the original temperature of said first gas. Other suitable cooling mediums which can be employed in the practice of the invention include ordinary cooling water at suitable temperatures, and any of the wellknown refrigerants such as ammonia, sulfur dioxide, Freon, and liquefied petroleum gases such as propane and butane.

The drawing is a schematic flow diagram illustrating an application of the invention to test heat exchanger in an ammonia oxidation plant.

Referring now to said drawing, the invention will be more fully'explained. Ammonia from line 10 enters mixer 11 wherein it is mixed with air from line 12 and heater 13. The air-ammonia mixture is passed via time 14 into ammonia converter 16. The ratio of ammonia to air can be controlled by an ammonia concentration analyzer-recorder-controller not shown. Air-ammonia mixtures containing less than 8 or more than 10.5 volume percent ammonia show a lower conversion efi'iciency and a preferred concentration of ammonia in the air-ammonia mixture is within the range of 9.5 to 10.3 volume percent.

In converter 16 ammonia is oxidized in accordance with the following equation:

The reaction is exothermic and it may start at a temperature as low as 500 C. Although higher and lower temperatures can be employed the most preferred temperature in converter 16 is usually within the range of 9009 l0 C. High pressures tend to increase the formation of nitrogen at the expense of the desired nitric oxides and therefore pressures in excess of 135 pounds per square inch gage are usually not employed. The temperature in converter 16 is maintained within the desired temperature range by controlling the preheat temperature of the air in heater 13 prior to its admixture with the ammonia. Any suitable temperature responsive device (not shown) can be employed for effecting this control.

Ammonia converter 16 is provided with a suitable catalyst for effecting the ammonia oxidation reaction. The preferred type of catalyst is a platinum cata'yst. Platinum alone or platinum in an alloy with a metal such as copper, nickel, cobalt, sulfur, tungsten, palladium, and the like can be used. A presently preferred catalyst is a platinum-rhodium alloy containing about 10 percent rhodium. This catalyst is usually employed in the form of an 80 mesh gauze with the gauze in either single or multiple layers.- However, any other suitable physical .4 form of the catalyst can be employed. Effluent from converter 16 contains about 10 percent by volume nitric oxide, about 7 percent by volume oxygen, about 14 to 15 volume percent steam, and nitrogen. Said efiluent from converter 16 is cooled by means not shown and passed via line 17 at a temperature Within the range of 1022 to 1202 F. (550-650 C.), usually about 600 G, into heat exchanger 18.

Heat exchanger 18 can be any of the many well-known types of heat exchanger. Said heat exchanger is most commonly a shelland tube type heat exchanger and can be any of the various types such as a multiple pass, single pass, floating head, etc. As here shown converter efiluent in line 17 is being passed through the tube side of said exchanger in indirect heat exchange relationship with absorber eiliuent gas from line 28 which is entering said heat exchanger on the shell side.

Said converter effluent gas or process gas is passed from heat exchanger 18 via line 19 at a temperature of about 440 C. and is passed through cooler 21 wherein its'temperature is lowered to no higher than 150 C. It is preferable to cool said gases to a temperature Within the range of 40 to C. Upon being cooled to a temperature no higher than 150 C., the nitric oxide in said converter effluent or process gas is oxidized by the oxygen contained therein in accordance with the following equation:

Said process gas from cooler 21 is passed via line 22 into absorber 23 wherein it is contacted with water introduced via line 24. Absorber 23 can be a bubble cap absorption column, a packed column or other suitable contact column. The water passes down through the column and reacts with nitrogen dioxide to form nitric acid and absorbs the nitric acids thus formed. Said nitric acid is withdrawn via line 25 and passed to. storage or other use. The nitric acid from absorber 23 usually contains some nitrogen trioxide (N 0 and consequently has a slightly brownish color. To remove this color the acid can be passed to a bleaching column not shown wherein it is contacted with air. If desired the air from said bleaching column, or other air, can be introduced into absorber 23 via line 35 to oxidize nitrous oxide in absorber 23. Absorber 23 is operated at the lowest temperature obtainable with ordinary cooling water which .is introduced through line 26 and passed through coils 27. Said temperature is usually within the range of 15 to 40 C. (59 to 104 F.). The pressure in absorber 23 is usually maintained within the range of 80 to pounds per square inch absolute.

Unabsorbed gases or absorber effluent from absorber 23 are withdrawn therefrom via line 28 and passed into the shell side of heat exchanger 18 in indirect heat exchange relationship with converter eflluent or process gas introduced into the tube side of said heat exchanger via ine 17. Tail gas is withdrawn from heat exchanger 18 via line 29 for disposal or other use as desired.

When it is desired to test heat exchanger 18 for the presence of a leak, a sample stream of said tail gas is withdrawn from line 29 and passed via line 31 into coil 32 of condenser 33. In condenser 33 said sample stream is cooled by indirect heat exchange by means of a stream of coolant introduced into said condenser via line 34 and withdrawn therefrom via line 36. As previously mentioned, said sample stream is cooled to a temperature substantially the same as, but not lower than, the dew point of the original absorber effluent gas, i.e., the gas in line 28. Since the absorber effluent gas in line 28 is saturated with Water vapor it is at its dew point with respect to water vapor.

Thus a particularly convenient coolant to employ in condenser 33 is a stream of the original absorber effluent gas from line .28 which can be passed into line 34 via line 37. However, if desired, other cooling mediums, mentioned above, can be introduced'via line 38. Condensate from condenser 33 is passed via line 39 into collection vessel 41. Said vessel 41 can be any suitable type of vessel. As here illustrated it has a swadged lower end which is fitted with a graduated glass tube 42 whereby the volume of condensate can be readily determined. Uncondensed gases pass upwardly in said collection vessel 41 around bafiles 43 and are withdrawn via line 44 for venting to the atmosphere. Pressure control valves 46 and 30 are employed to maintain the desired back pressure on said collection vessel 41 and the remainder of the system. If desired, temperature recorder-controller 50 can be employed to actuate motor valve 51 responsive to thermocouple 52 so as to maintain the temperature of the gas exiting from vessel 41 via line 44 at a predetermined desired temperature. As shown, said temperature recorder-controller 50 can be operatively connected to line 28 so as to reset the control point of said TRC 50 in accordance with the temperature in said line 28 and thus hold the temperature of the gases leaving vessel 41 substantially the same as the temperature of the gases in lines 28.

The following example will serve to further illustrate the invention. Y i

' Example [5330 standard cubic feet per minute' (890 mols per hour) of converter effluent or process gas from converter 16 and having a composition in volume percent of 9.4 percent NO, 6.4 percent 0 14.6 percent water vapor, and 69.6 percent N is passed at a temperature of 600 C. and under a pressure of 96 pounds per square inch absolute through the tube side of heat exchanger 18 in indirect heat exchange with 784 mols per hour of absorber efiluent gas from line 28 under a pressure of 93 pounds per square inch absolute and at a temperature of 70 F. (21.l C.). Said absorber efiluent gas is passed through the shell side of heat'exchanger 18. A sample stream consisting of 6.4 standard cubic feet per minute of the exit tail gas from heat exchanger 18 is withdrawn from line 29 and passed via line 31 into the tube side of condenser 33. In condenser 33 said sample stream of gas is cooled to a temperature of 73 F. (22.8 C.) by indirect heat exchange. with a stream of water introduced through line 38. The

cooled gas is passed through collection vessel 41 and vented through line 44. Pressure control valve maintains a pressure of 91 p.s.i.a. in line 44. After one hour ccs. of Water had collected in graduated tube 42. Based on the water content of said process gas stream and said absorber efiluent gas stream it was calculated that about 1 percent by volume of process gas passing through the tube side of heat exchanger 18 was leaking into absorber eflluent gas passing through the shell side of said heat exchanger 18. Upon shutdown and inspection a crack was found in one of the tube sheets of heat exchanger 18. Laboratory analysis of the tail gas stream in line 29 had failed to indicate the presence of a leak.

The value of applicants invention will be readily apparent to those skilled in the art in view of this disclosure. For example, in the ammonia oxidation process described, a leak of the process gas from the tube side of heat exchanger 18 into the shell side of said heat exchanger means that said leaked process gas does not pass through absorber 23 and is therefore lost insofar as the production of nitric acid is concerned. There is, therefore, a drop in conversion efficiency of the plant which can result in substantial economic loss. Prior to our invention when such a leak occurred it was customary to first check the activity of the catalyst in converter 16. When a check on the catalyst activity indicated that the catalyst possessed normal activity the heat exchange system was checked for leaks.

Detection of a leak from the tube side of heat exchanger 18 to the shell side of said heat exchanger is diflicult by quantitative laboratory analysis because of thesimilarity in compositionof the two gas streams. Frequently, even though an appreciable leak is present, laboratory analysis will fail to indicate the presence of the leak. Thus, prior to our invention when a check on the catalyst activity showed the catalyst to possess normal activity, and laboratory analysis of the tail gas in line 29 from heat exchanger 18 failed to indicate the presence of a leak when there actually was a leak, the reason for the above mentioned drop in conversion efiiciency was obscured and much time and efiort were lost in searching for the cause for said drop in conversion efficiency.

Our invention makes it possible, when a drop in conversion eificiency occurs, to readily and conveniently check the heat exchange system for leaks. The method of testing disclosed herein has been found to be accurate and reliable. The availability of such a method of testing has resulted in considerable savings of manpower and production.

While the invention has been described as applied to testing heat exchange equipmentin an ammonia oxidation process, the invention is not to be limited thereto. The invention can be applied to the testing of any heat exchange system wherein two gases are passed in indirect heat exchange relationship.

For'example, in the synthesis of ammonia from nitrogen and hydrogen, the converter eflluent is passed through a heat exchanger and then through a condenser so as to condense the ammonia product which is collected in a receiver. Unconverted nitrogen and hydrogen are withdrawn from said receiver, passed through said heat exchanger in indirect heat exchange with said converter efiluent, and then recycled to the converter. The method of the invention can be employed to test the stream of unconverted nitrogen and hydrogen exiting from said heat exchanger for the presence of ammonia and thus detect any leak which may exist in said heat exchanger.

' Similarly, in a process for the dehydrogenation of butenes to butadiene, effluent from the dehydrogenation reactor is first quenched with water and is then passed through a heat exchanger in indirect heat exchange with air which is to be employed for combustion purposes. A sample of the air stream exiting from said heat exchanger can be tested according to the invention for the presence of water vapor to detect any leaks which may exist in said heat exchanger.

Various other modifications of the invention will be apparent to those skilled in the art in view of the above disclosure. Such modifications are within the spirit and scope of the invention.

We claim:

1. In a process wherein a cool first gas having a known dew point with respect to a readily eondensible component is passed through a heat exchanger in indirect heat exchange with a warm second gas under a greater pressure and containing a higher concentration of said readily eondensible component than said cool first gas, the method of detecting a leak in said heat exchanger, which method comprises: passing a sample stream of said first gas exiting from said heat exchanger through a condenser in indirect heat exchange with a stream of said cool first gas withdrawn from the system at a point prior to its entry into said heat exchanger and cooling said sample stream to a temperature substantially the same as but not lower than said dew point of said first gas prior to entering said heat exchanger; maintaining the pressure of said sample stream during said cooling at a value substantially the same as but not greater than the original pressure of said first gas prior to entering said heat ex-' changer; and collecting condensate of said readily condensible component.

2. A method according to claim 1 wherein said readily eondensible component is water vapor.

3. In a process wherein a cool first gas having a known dew point with respect to a readily condensible compomanor;

7 nent is passed through a heat exchanger in indirect heat exchange with a warm second gas under a greater pres sure and containing a higher concentration of said readily condensible component than said first gas, the method of determining the magnitude of a leak in said heat exchanger, which method comprises: passing a stream of said first gas through said heat exchanger at a predetermined temperature, pressure, and rate of flow; simultaneously passing a stream of said second gas containing a known amount of said readilycondensible component through said heat exchanger at a predetermined temperature, pressure, and rate of flow; passing a sample stream of said first gas exiting from said heat exchanger through a condenser, at a predetermined temperature, pressure, and rate of flow, in indirect heat exchange with a stream of said cool first gas Withdrawn from tl e system at a point prior to its entry into said heat exchanger and cooling said sample stream to a temperature substantially the same as but not lower than said dew point of said first gas prior to entering said heat exchanger; maintaining the pressure of said sample stream during said cooling at a value substantially the same as but not greater than the original pressure of said first gas prior to entering said heat exchanger; and measuring the amount of condensate of said readily condensibie component.

4. A method according to claim 3 wherein said readily condensible component is water vapor.

5. In a process for the production of nitric acid wherein an absorber efiluent gas consisting essentially of nitrogen, oxygen, and saturated with water vapor is passed at a temperature within the range of 59 to 104 F. and a pressure within the range of 80 to 110 p.s.i.a. through a heat exchanger in indirect heat exchange relationship with a process gas stream under a pressure of 83 to 113 p.s.i.a. and at a temperature of 1022 to 1202 F. and consisting essentially of nitrogen oxides, oxygen, nitrogen, and from 14 to 15 volume percent water vapor, the method of detecting a leak in said heat exchanger, which method comprises: passing a stream of said absorber efiluent gas through said heat exchanger at a predetermined temperature, pressure, and rate of flow; simultaneously passing a stream of said process gas through said heat exchanger at a predetermined temperature, a predetermined pressure greater than said predetermined pressure on said stream of absorber efiiuent, and a predetermined rate of flow; passing a sample stream of said absorber efliuent gas exiting from said heat exchanger through a condenser at a predetermined, temperature, pressure, and rate of flow in indirect heat exchange With a stream of said absorber efiluent gas withdrawn from the system at a point prior to its entry into said heat exchanger and cooling said sample stream to a temperature within the range of from O ,to 5 degrees F. greater than the original temperature of said absorber efiiuent gas prior to entering said heat exchanger; maintaining the pressure of said sample stream during said cooling in said condenser at a value within the range of from 0 to 5 pounds less than the pressure of said absorber etfiuent gas prior to entering said heat exchanger; and collecting water vapor condensed in said condenser.

6. In a process for the production of nitric acid wherein an absorber etfiuent gas consisting essentially of nitrogen, oxygen, and saturated with water vapor is passed at a temperature within the range of 59 to 104 F. and a pressure within the range of to p.s.i.a. through a heat exchanger in indirect heat exchange relationship with a process gas stream under a pressure of 83 to 113 p.s.i.a. and at a temperature of 1022 to 1202 F., and consisting essentially of nitrogen oxides, oxygen, nitrogen, and from 14 to 15 volume percent water vapor, the method of determining the magnitude of a leak in said heat exchanger, which method comprises: passing a stream of said absorber efiluent gas through said heat exchanger at a predetermined temperature, pressure, and rate of flow; simultaneously passing a stream of said process gas through said heat exchanger at a predetermined temperature, a predetermined pressure greater than said predetermined pressure on said stream of absorber cffiuent, and a predetermined rate of flow; passing a sample stream of said absorber efiluent gas exiting from said heat exchanger through a condenser at a predetermined temperature, pressure, and rate of flow in indirect heat exchange with a stream of said absorber effiuent gas withdrawn from the system at a point prior to its entry into said heat exchanger and cooling said sample stream to a temperature within the range of from 0 to 5 degrees F. greater than the original temperature of said absorber etiiuent gas prior to entering said heat exchanger; maintaining the pressure of said sample stream during said'cooling in said condenser at a value Within the range of from 0 to 5 pounds less than the pressure of said absorber efiluent gas prior to entering said heat exchanger; and measuring the amount of water vapor condensed in said condenser.

References Cited in the file of this patent UNITED STATES PATENTS 2,169,826 Wendlandt Aug. 15, 1939 2,658,728 Evan's Nov. 10, 1953 I FOREIGN PATENTS I 748,264 Great Britain Apr. 25, 1956 

1. IN A PROCESS WHEREIN A COOL FIRST GAS HAVING A KNOWN DEW POINT WITH RESPECT TO A READILY CONDENSIBLE COMPONENT IS PASSED THROUGH A HEAT EXCHANGER IN INDIRECT HEAT EXCHANGE WITH A WARM SECOND GAS UNDER A GREATER PRESSURE AND CONTAINING A HIGHER CONCENTRATION OF SAID READILY CONDENSIBLE COMPONENT THAN SAID COOL FIRST GAS, THE METHOD OF DETECTING A LEAK IN SAID HEAT EXCHANGER, WHICH METHOD COMPRISES: PASSING A SAMPLE STREAM OF SAID FIRST GAS EXITING FROM SAID HEAT EXCHANGER THROUGH A CONDENSER IN INDIRECT HEAT EXCHANGE WITH A STREAM OF SAID COOL FIRST GAS WITHDRAWN FROM THE SYSTEM AT A POINT PRIOR TO ITS ENTRY INTO SAID HEAT EXCHANGER AND COOLING SAID SAMPLE STREAM TO A TEMPERATURE SUBSTANTIALLY THE SAME AS BUT NOT LOWER THAN SAID DEW POINT OF SAID FIRST GAS PRIOR TO ENTERING SAID HEAT EXCHANGER, MAINTAINING THE PRESSURE OF SAID SAMPLE STREAM DURING SAID COOLING AT A VALUE SUBSTANTIALLY THE SAME AS BUT NOT GREATER THAN THE ORIGINAL 